Fibroblast growth factor (FGF) 23, is an endocrine regulator of phosphate homeostasis and vitamin D metabolism, and was originally identified as the mutated gene in patients with the phosphate wasting disorder “autosomal dominant hypophosphatemic rickets” (ADHR) (Anonymous, “Autosomal Dominant Hypophosphataemic Rickets is Associated with Mutations in FGF23,” Nat Genet 26(3):345-348 (2000)). FGF23 inhibits reabsorption of phosphate in the renal proximal tubule by decreasing the abundance of the type II sodium-dependent phosphate transporters NaPi-2A and NaPi-2C in the apical brush border membrane (Baum et al., “Effect of Fibroblast Growth Factor-23 on Phosphate Transport in Proximal Tubules,” Kidney Int 68(3):1148-1153 (2005); Perwad et al., “Fibroblast Growth Factor 23 Impairs Phosphorus and Vitamin D Metabolism In Vivo and Suppresses 25-hydroxyvitamin D-1alpha-hydroxylase Expression In Vitro,” Am J Physiol Renal Physiol 293(5):F1577-1583 (2007); Larsson et al., “Transgenic Mice Expressing Fibroblast Growth Factor 23 under the Control of the Alpha1(I) Collagen Promoter Exhibit Growth Retardation, Osteomalacia, and Disturbed Phosphate Homeostasis,” Endocrinology 145(7):3087-3094 (2004)). The phosphaturic activity of FGF23 is down-regulated by proteolytic cleavage at the 176RXXR179 (SEQ ID NO: 233) motif, where “XX” is defined as “HT”, corresponding to positions 177 and 178, respectively, of the FGF23 amino acid sequence, producing an inactive N-terminal fragment (Y25 to R179) and a C-terminal fragment (S180 to 1251) (Goetz et al., “Molecular Insights into the Klotho-dependent, Endocrine Mode of Action of Fibroblast Growth Factor 19 Subfamily Members,” Mol Cell Biol 27(9):3417-3428 (2007)). αKlotho, a protein first described as an aging suppressor (Kuro-o et al., “Mutation of the Mouse Klotho Gene Leads to a Syndrome Resembling Aging,” Nature 390(6655):45-51 (1997)), is required by FGF23 in its target tissue in order to exert its phosphaturic activity (Kurosu et al., “Regulation of Fibroblast Growth Factor-23 Signaling by Klotho,” J Biol Chem 281(10):6120-6123 (2006); Urakawa et al., “Klotho Converts Canonical FGF Receptor into a Specific Receptor for FGF23,” Nature 444(7120):770-774 (2006)). αKlotho constitutively binds the cognate FGFRs of FGF23, and the binary FGFR-αKlotho complexes exhibit enhanced binding affinity for FGF23 ((Kurosu et al., “Regulation of Fibroblast Growth Factor-23 Signaling by Klotho,” J Biol Chem 281(10):6120-6123 (2006); Urakawa et al., “Klotho Converts Canonical FGF Receptor into a Specific Receptor for FGF23,” Nature 444(7120):770-774 (2006)). In co-immunoprecipitation studies, it was demonstrated that the mature, full-length form of FGF23 (Y25 to 1251) but not the inactive N-terminal fragment of proteolytic cleavage (Y25 to R179) binds to binary FGFR-αKlotho complexes (Goetz et al., “Molecular Insights into the Klotho-dependent, Endocrine Mode of Action of Fibroblast Growth Factor 19 Subfamily Members,” Mol Cell Biol 27(9):3417-3428 (2007)).
It was further shown that the mature, full-length form of FGF23 (Y25 to I251) forms a stable ternary complex with the ectodomain of αKlotho and the ligand-binding domain of FGFR1c in solution (Goetz et al., “Isolated C-terminal Tail of FGF23 Alleviates Hypophosphatemia by Inhibiting FGF23-FGFR-Klotho Complex Formation,” Proc Natl Acad Sci USA 107:407-412 (2010)). The ligand interacts with a de novo binding site generated at the composite receptor-coreceptor interface in the binary αKlotho-FGFR complex (Goetz et al., “Isolated C-terminal Tail of FGF23 Alleviates Hypophosphatemia by Inhibiting FGF23-FGFR-Klotho Complex Formation,” Proc Natl Acad Sci USA 107:407-412 (2010)). The region on FGF23 that binds to this de novo site was mapped to the 72 amino acid long C-terminal tail, which follows the β-trefoil core domain (Goetz et al., “Isolated C-terminal Tail of FGF23 Alleviates Hypophosphatemia by Inhibiting FGF23-FGFR-Klotho Complex Formation,” Proc Natl Acad Sci USA 107:407-412 (2010)). Thus, the N-terminal fragment of proteolytic cleavage (Y25 to R179) is metabolically inactive because it lacks the binding site for the αKlotho-FGFR complex. The C-terminal proteolytic fragment (S180 to I251), however, can compete with full-length FGF23 for binding to the αKlotho-FGFR complex to antagonize the metabolic activity of FGF23, because this fragment contains the binding site for the αKlotho-FGFR complex (Goetz et al., “Isolated C-terminal Tail of FGF23 Alleviates Hypophosphatemia by Inhibiting FGF23-FGFR-Klotho Complex Formation,” Proc Natl Acad Sci USA 107:407-412 (2010)). These findings suggest a dual mechanism by which proteolytic cleavage at the RXXR motif inactivates FGF23: the cleavage removes the binding site for the αKlotho-FGFR complex from FGF23 and concomitantly generates an endogenous inhibitor of FGF23. Inhibition of this proteolytic cleavage by missense mutations at the RXXR motif in FGF23 leads to accumulation of full-length, bioactive FGF23, causing renal phosphate wasting disease in humans (Shimada et al., “Mutant FGF-23 Responsible for Autosomal Dominant Hypophosphatemic Rickets is Resistant to Proteolytic Cleavage and Causes Hypophosphatemia in vivo,” Endocrinology 143:3179-3182 (2002); White et al., “Autosomal-dominant Hypophosphatemic Rickets (ADHR) Mutations Stabilize FGF-23,” Kidney Int 60:2079-2086 (2001); White et al., “Autosomal Dominant Hypophosphataemic Rickets is Associated with Mutations in FGF23,” Nature Genet 26:345-348 (2000)).
Conversely, enhanced FGF23 cleavage due to impaired O-glycosylation of FGF23 leads to a deficit in full-length FGF23, which manifests as hyperphosphatemia and soft tissue calcification in humans (Frishberg Y et al., “Hyperostosis-hyperphosphatemia Syndrome: a Congenital Disorder of O-glycosylation Associated with Augmented Processing of Fibroblast Growth Factor 23,” J Bone Miner Res 22:235-242 (2007); Kato et al., “Polypeptide GalNAc-transferase T3 and Familial Tumoral Calcinosis. Secretion of Fibroblast Growth Factor 23 Requires O-glycosylation,” J Biol Chem 281:18370-18377 (2006)). Familial tumoral calcinosis is an autosomal recessive metabolic disorder associated with hyperphosphatemia and soft tissue calcification. Missense mutations in either the UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylglactosaminyltransferase 3 (GALNT3) gene (Garringer et al., “Two Novel GALNT3 Mutations in Familial Tumoral Calcinosis,” Am J Med Genet A 143A:2390-2396 (2007)) or the FGF23 gene (Garringer et al., “Molecular Genetic and Biochemical Analyses of FGF23 Mutations in Familial Tumoral Calcinosis,” Am J Physiol Endocrinol Metab 295:E929-E937 (2008); Araya et al., “A Novel Mutation in Fibroblast Growth Factor 23 Gene as a Cause of Tumoral Calcinosis,” J Clin Endocrinol Metab 90:5523-5527 (2005)) have been associated with familial tumoral calcinosis. There is a great need for suitable treatments for such patients.
The present invention is directed to overcoming these and other deficiencies in the art.